I'm restoring an old World War 1 bayonet, about 10" long. How much would it cost to have the blade re-chromed?

Upon further investigation, it turns out to be pre-WW1

I doubt it would ever have been chromed! If its rusty try soaking it over night in cheap coke (keep it off the 'handle' if wetness would damage it though, you could put it in a basin and wrap it in coke soaked paper maybe. You may find if you are planning on selling it that it is better not to clean it up - some collectors prefer the original as it is. But coke will get off rust - its got phosphoric acid in it and blacksmiths use this to clean up their metal